The present invention relates generally to the output power control of eye safe structured laser light. Such structured laser light is used to illuminate features which are then imaged with a one-dimensional or two-dimensional imaging array (camera). In particular, the present invention takes advantage of laser safety standards which allow for greater peak power levels for pulsed lasers. Particularly, in applications requiring high speed xe2x80x9cstop actionxe2x80x9d imaging the present invention will, transparent to the user, facilitate optimization for best signal level at selected integration times.
Traditionally, for any commercial laser projector systems, laser output power is set at a single fixed xe2x80x9csafexe2x80x9d power level during manufacture of the laser projection device. The laser power setting for commercial laser products is established per applicable U.S. and international safety standards. For example, CDRH 21 CFR Part 1040.10 requires that commercial CW laser systems rated at Class II emit no more than 1 milliwatt of visible laser power into a 7 millimeter aperture at a specified distance from the laser source. For pulsed laser systems, the same 1 mW power level rating is in effect, but the power level is now considered average rather than continuous power. Average power for a pulsed laser is computed as the peak pulse power multiplied by the duty cycle (percentage of on-time of the pulse) of the pulse train.
For pulsed laser systems, international laser safety standards also allow for greater peak pulse power levels as long as the 1 mW average power setting is not violated. A formula for visible laser radiation and appropriate to the intended use of the laser establishes the maximum allowable peak power. In particular, EN 60825-1 standard for Class 3A rated laser systems requires that the following formula for visible light with pulse duration greater than 1.8 microseconds be used to establish the maximum peak power level using the 7 mm aperture:                     Pp        =                  0.7                                    (                              N                *                                  t                  p                                            )                        0.25                                                            where          ⁢                      :                    ⁢                      xe2x80x83                    ⁢          Pp                =                  peak          ⁢                      xe2x80x83                    ⁢          pulse          ⁢                      xe2x80x83                    ⁢          power          ⁢                      xe2x80x83                    ⁢          in          ⁢                      xe2x80x83                    ⁢          mW                                                  N          =                      number            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            pulses            ⁢                          xe2x80x83                        ⁢            in            ⁢                          xe2x80x83                        ⁢            0.25            ⁢                          xe2x80x83                        ⁢            second            ⁢                          xe2x80x83                        ⁢            interval                          ⁢                  xe2x80x83                                                  t          p                =                  pulse          ⁢                      xe2x80x83                    ⁢          duration          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          an          ⁢                      xe2x80x83                    ⁢          individual          ⁢                      xe2x80x83                    ⁢          pulse          ⁢                      xe2x80x83                    ⁢          in          ⁢                      xe2x80x83                    ⁢          seconds                    
In this case, depending on the pulse duration and pulse duty cycle, it is possible to have a peak pulse power level much greater than the CW power setting. For example, for a pulse duration of 0.001 seconds and N=1 pulse train (one pulse every 0.25 seconds), the maximum allowed peak power would be almost 4 mW with a consequential average power of approximately 0.016 mW.
Laser safety standards require that adequate safeguards be designed into the laser drive circuitry to ensure that safe power settings are not exceeded. This is usually implemented in commercial CW laser projectors using a potentiometer which is adjusted to the proper resistance setting resulting in a value less than 1 mW output power into the specified aperture. In the case of pulsed laser projectors, the output power is normally set at a fixed level corresponding to the maximum peak power level at a particular pulse duration at a fixed pulse clock frequency. Analog circuitry is used to monitor the pulse signal to ensure that the pulse parameters are maintained within safe limits.
For applications such as the imaging of reflected structured laser light pulsing of the laser projector has been used to improve xe2x80x9cstop-actionxe2x80x9d imaging of moving targets of interest. Stop-action or strobing of the target by the laser projector is used to avoid blurring resulting from the relative motion during the imager integration period. The application of strobing over short durations also results in very low relative signal levels due to the short camera exposure intervals of the imager. This problem is exaggerated if the targets of interest have very low optical reflectance properties thus giving extremely low signal levels acquired by the imager during the exposure time.
The present invention provides a method and system for incorporating into a laser driver circuit a technique to automatically maximize the operating parameters of a laser diode within the constraints of a predetermined standard. The laser diode operating characteristic is based on the laser diode gain which is determined during calibration of each laser diode projector as well as application of the specified laser safety formulas.
The present laser control system provides a system and method for controlling the operating parameters of a laser diode. The preferred embodiment of the laser control circuit includes a laser diode that is powered by a laser drive current. The laser diode has a laser output having a peak power level. A detector is coupled to the laser diode for sensing the laser output. A laser driver including a primary control loop is operable, in response to the sensed laser output and a reference, to control the laser drive current such that the output characteristic corresponds to the reference. A controller is coupled to the driver. The controller includes a laser settings module for generating the reference in response to a laser output setting such that the laser output characteristic is maintained at approximately a predetermined output level. The output level of the laser diode is maintained within a predetermined safety standard. Another aspect of the invention provides an independent safety monitoring function based on the laser settings.